1. Field of the Invention
The invention relates to a four-wheel-drive vehicle, and a control unit for the four-wheel-drive vehicle.
2. Description of Related Art
In some conventional four-wheel-drive vehicles, transmission of torque is interrupted on both input and output sides of a propeller shaft to prevent rotation of the propeller shaft while the vehicle is travelling in a two-wheel-drive mode, in order to improve the fuel efficiency by reducing power loss caused by the rotation of the propeller shaft while the vehicle is travelling in the two-wheel-drive mode (refer to, for example, Japanese Patent Application Publication No. 2003-220847 (JP 2003-220847 A)).
The four-wheel-drive vehicle described in Japanese Patent Application Publication No. 2003-220847 has a transfer clutch and an automatic disconnecting differential (ADD) mechanism. The transfer clutch is provided in a transfer unit, and the ADD mechanism is arranged between a differential gear unit on auxiliary drive wheel (right and left front wheels) side and one (right front wheel) of the auxiliary drive wheels. The transfer clutch is a hydraulic clutch or an electromagnetic clutch capable of variably controlling the engagement torque. The ADD mechanism is able to switch the state of connection between an output portion of the differential gear unit and a drive shaft on the auxiliary drive wheel side, between a lock state in which the output portion and the drive shaft are connected to each other and a free state in which the output portion and the drive shaft are disconnected from each other.
The transfer clutch and the ADD mechanism are controlled by a four-wheel-drive controller that is capable of switching the drive mode between a two-wheel-drive mode in which only main drive wheels (right and left rear wheels) are driven, and a four-wheel-drive mode in which the main drive wheels and the auxiliary drive wheels are both driven. In the two-wheel-drive mode, transmission of torque by the transfer clutch and the ADD mechanism is interrupted to stop the rotation of a front propeller shaft. In order to switch the drive mode from the two-wheel-drive mode to the four-wheel-drive mode, the transfer clutch is engaged with a predetermined torque required to start the rotation of the front propeller shaft. After that, when it is determined that the input rotational speed and output rotational speed of the ADD mechanism are synchronized with each other, the ADD mechanism switches the connection state from the free state to the lock state.
In order to smoothly synchronize the rotation of the propeller shaft upon switchover from the two-wheel-drive mode to the four-wheel-drive mode, it is desirable to set the torque for rotating the propeller shaft to a high value. However, the inventor of the present invention has confirmed that if the torque is excessively high, vibrations occur at a distal end portion of the propeller shaft due to torsion of the propeller shaft. If vibrations occur at the distal end portion of the propeller shaft, the vibrations may be transmitted to a driver and a passenger and give them a sense of discomfort and it may take a long time to synchronize the rotation of the propeller shaft.